1. Field of the Invention
The present invention relates to phase shifting masks and, in particular, to the use of model-based tools to facilitate phase assignment on the phase shifting masks.
2. Discussion of the Related Art
Lithography is a well-known process used in the semiconductor industry to form lines, contacts, and other known structures in integrated circuits (ICs). In conventional lithography, a mask (wherein the term xe2x80x9cmaskxe2x80x9d as used herein can refer to a mask or a reticle) having a pattern of transparent and opaque regions representing such structures in one IC layer is illuminated. The emanating light from the mask is then focused onto a photoresist layer provided on a wafer. During a subsequent development process, portions of the photoresist layer are removed, wherein the portions are defined by the pattern. In this manner, the pattern of the mask is transferred to (i.e. printed on) the photoresist layer.
However, diffraction effects at the transition of the transparent regions to the opaque regions on the mask can render the corresponding printed edges on the wafer indistinct, thereby adversely affecting the resolution of the lithography process. Various techniques have been proposed to improve the resolution. One such technique, phase shifting, uses phase destructive interference of the waves of incident light. Specifically, phase shifting shifts the phase of a first region of incident light waves approximately 180 degrees relative to a second, adjacent region of incident light waves to create a feature between the first and second regions. Thus, a feature, as defined by exposed and unexposed portions of a photoresist illuminated through a mask, can be more closely defined by using phase shifting, thereby allowing greater structure density on the IC. Typically, features generated by phase shifting can then be protected from exposure by using a xe2x80x9ctrimxe2x80x9d mask, which is used to expose the remaining field.
As the need for feature density increases, phase shifting is being applied to many features on the layout. In one embodiment, called a full phase approach, substantially all features of a layer can be defined using phase shifting. However, using phase shifting in dense layouts can result in phase conflicts. Phase conflicts can negate the optical interference necessary to create the desired feature(s). Therefore, assigning phase to the layout can constitute a time-intensive, but mandatory part of typical process flows in the production of many integrated circuits.
Other modifications can be made to the layout to optimize printing resolution. For example, a process called optical proximity correction (OPC) can be used to compensate for non-linear distortions caused by optical diffraction and resist process effects when transferring the pattern from the mask to the wafer. Advanced computer-implemented tools can simulate (i.e. predict) a real pattern transfer with a set of mathematical formulas (i.e. models). In simulating pattern transfer, a simulation tool can use one or multiple models with the layout to generate an OPC-modified layout.
FIG. 1 illustrates a standard process flow 100 including phase assignment as well as OPC. In step 101 of process flow 100, a target layout can be provided to a phase shifting tool for analysis. In one embodiment, the iN-Phase(trademark) tool, licensed by Numerical Technologies, Inc., can be used. The PSM type, e.g. double exposure alternating aperture phase shifting, can be designated in step 102. Step 103, in which phase shifting is applied to the target layout, can include the steps of placing the shifters in the layout (step 104), assigning phase to the shifters (step 105), and resolving any phase conflicts (step 106). Step 105 may include algorithms for assigning phases to the shifters.
The algorithms used are based phase dependencies and, optionally, costs. By describing the dependencies in one or more data structures, the algorithms can attempt to solve the problem. See, e.g., U.S. patent application Ser. No. 09/823,380, filed Mar. 29, 2001, entitled, xe2x80x9cIncrementally Resolved Phase-shift Conflicts In Layouts For Phase-Shifted Featuresxe2x80x9d, and U.S. patent application Ser. No. 10/085,759, filed Feb. 28, 2002, entitled xe2x80x9cDesign And Layout Of Phase Shifting Photolithographic Masksxe2x80x9d, both of which are incorporated by reference herein. Performing these algorithms to assign phase can be extremely time consuming and require a separate computer program and/or software to be maintained for performing phase assignment, as opposed to the simulation engine for model-based optical proximity correction.
After being output in step 107, the phase shifted layout can then be input to a tool that applies OPC. In one embodiment, the tool for applying phase shifting is different than the tool for providing OPC. In another embodiment, the same tool can provide both phase shifting and OPC. For example, the iN-Phase(trademark) tool, licensed by Numerical Technologies, Inc., can assign phase to shifters as well as perform OPC of a finished layout. In step 109, the layout can be modified as appropriate. At this point, the final layout can be output in step 110. Unfortunately, in either embodiment, significant time is spent reading (or streaming out) and then streaming in the phase shifted layout (steps 107 and 108).
Therefore, a need arises for an accurate and time effective method and system for assigning phase to shifters in a model-based environment.
In accordance with one feature of the invention, shifters on a phase shifting mask (PSM) can be assigned their corresponding phase through the use of a simulation engine of the type normally used for model-based optical proximity correction (OPC). Specifically, instead of using an algorithm based on phase dependencies, the phase of a shifter can be assigned based on simulating the image contrast provided by each phase for that shifter. The higher the contrast, the better the lithographic performance of the shifter. Therefore, the phase providing the higher contrast can be selected for that shifter.
To provide this simulation-based phase assignment, a pre-shifter, or shifter shape, can be placed relative to a feature on the layout. In one embodiment, the pre-shifter can be placed close to all edges of the feature, thereby surrounding the feature. The pre-shifter can then be divided into a plurality of shifter tiles along the edges where the shifters abut the feature. This division can be done using an OPC engine""s dissection capabilities. A first contrast and a second contrast can be calculated by assigning a first phase and a second phase, respectively, to a shifter tile. In multi-phase phase shifting masks, additional contrasts can be computed for each phase possibility. A phase for the shifter tile can be selected based on whichever contrast is higher (and will more accurately produce the feature at the target dimension). These steps can be repeated for multiple shifter tiles for the feature as well as other features on the layout. In another embodiment, all the evaluation points can be calculated at the same time using the 0 degree/180 degree options to determine which option give the best contrast.
In one embodiment, the pre-shifter can be cut at predetermined areas of the feature. For example, the predetermined areas can include corners of the feature. In this manner, phase conflicts can be avoided. In another embodiment, cuts are not made to the pre-shifter at line ends. Specifically, a trim mask (which will be used in conjunction with the PSM) can be used to define the line end, thereby rendering shifters unnecessary for this region.
The trim mask includes a plurality of trim regions for protecting the features defined by the PSM. In accordance with another feature of the invention, a trim region can be placed relative to a feature on the layout. This trim region, like the pre-shifter, can be divided into a plurality of trim tiles. In one embodiment, the dividing of the pre-shifter and the trim region can be based on dissection points provided on the feature. Note that these trim tiles can be shaped in response to the cuts made to the pre-shifter.
After phases for the shifter tiles are assigned using image contrast comparison, adjacent shifter tiles having identical phase can be merged, thereby improving the manufacturability of the PSM. The trim tiles corresponding to the merged adjacent shifter tiles can also be merged, thereby also improving the manufacturability of the trim mask.
In one embodiment, after or during phase selection for the plurality of shifter tiles, the process can be repeated for that feature, thereby allowing some fine-tuning of shifter phase. In another embodiment, the effect of optical proximity correction (OPC) associated with at least one shifter tile can be analyzed. Note that OPC can also advantageously ensure that a valid comparison is made between the two image contrast simulations. For example, for each tile, the contrast of the image with OPC can be calculated. Based on the contrast, the tile can be assigned a phase. After all tiles are assigned a phase, the OPC can be performed. In another embodiment, the OPC of a tile could be performed after phase assignment of that tile. After phase assignment and OPC of all tiles, additional OPC iterations could be performed, as necessary.
In accordance with one feature of the invention, the simulation-based assignment of phase to shifters can be performed using model-based data conversion. Model-based data conversion provides significant functionality and control mechanisms. For example, the dissection points provided on the feature (e.g. previously used solely for OPC) can be advantageously used for both phase assignment as well as OPC.
In this manner, processes that would previously be performed separately, thereby requiring a streaming out of data and then a subsequent streaming in of data (by either the same or even different tools), can be integrated into a comprehensive process. Thus, model-based data conversion can significantly reduce the total time associated with performing phase assignment and OPC.
Moreover, because of the significant functionality and control mechanisms associated with model-based data conversion, a user can avoid an uncorrectable layout. Specifically, the image contrast comparison reflects an objective standard that can be used to judge the lithographic performance of a shifter. Moreover, phase shifting and OPC can occur sequentially or in conjunction with one another, thereby allowing modifications to the layout that could not be done before the model-based data conversion approach.
Embodiments of the invention can include computer software for placing the pre-shifters around the features, dividing the pre-shifters into shifter tiles, determining the contrasts provided by the simulation images using different phases, and selecting the appropriate phases for the shifters. The computer software can include a source code segment for dividing the pre-shifters using dissection points. The computer software can further include a source code segment for dividing the shifter tiles into shifter segments, thereby allowing even more detailed analysis to be performed. A source code segment can also be provided for grouping a set of adjacent shifter tiles having a same phase to simplify manufacturing of the PSM. Finally, a source code segment can be provided for including OPC in at least one shifter tile.
Embodiments of the invention can also include a photolithographic mask manufactured with a layout generated using model-based data conversion. The photolithographic mask can include a dark field mask having a plurality of shifters. The following process can define the shifters. A pre-shifter can be placed proximate to the edges of a feature on the layout. The pre-shifter can be divided into a plurality of shifter tiles. First and second contrasts provided by assigning first and second phases, respectively, to a shifter tile can be determined. A phase for the shifter tile can be selected based on the step of determining. The steps of determining the contrasts and selecting the phase can be repeated for other shifter tiles. In one embodiment, the effect of optical proximity correction (OPC) for one or more shifter tiles can be analyzed in conjunction with phase assignment. The process can be repeated for other features.
Embodiments of the invention can include a tool for defining a layout for a phase shifting mask (PSM). The tool can include means for comparing a first contrast provided by assigning a first phase to a shifter on the layout and a second contrast provided by assigning a second phase to the shifter, means for selecting whichever of the first and second phases provides an overall better contrast, and means for repeating the steps of comparing and selecting for multiple shifters on the layout. The tool can further include means for providing optical proximity correction (OPC) associated with at least one shifter as well as means for modifying characteristics of that shifter. In one embodiment, the means for comparing and providing perform simulations. Of importance, the means for repeating and providing can be structured without streaming there between.
In another embodiment of the invention, process latitude can replace contrast of the image as the metric. For example, a critical dimension (CD) error range could be compared using the same predetermined dose and focus latitude for each phase, thereby determining which phase would minimize the CD error range. In another example, the range of process latitude for a given CD tolerance change could be computed for both phases, thereby identifying the phase that results in the greater process latitude.